Materials having good transparency, refractive index, mechanical strengths such as tensile strength and elongation at break, and heat and light resistances are needed for encapsulating materials for LEDs, and lens. Epoxy resins, poly(meth)acrylates and polycarbonates have mostly been used in the prior art. However, heat resistances and discoloration resistances of these resins are insufficient in conditions of high power of LED light-emitting devices and high temperatures of a long period of time.
When LEDs are soldered to substrates, lead-free solders are mostly used recently. The lead-free solders have higher melting temperatures than old-type solders and, therefore, soldering is conducted usually at a temperature of 260 degrees C. or higher. Then, the previous encapsulating materials and lens made of the thermoplastic resin sometimes change their shapes, or turn yellow at such a high temperature.
Then, compositions comprising silicones resins, called MQ resins, composed of M units and Q units were proposed in the following patent literatures 1, 2 and 3. Cured products of these compositions have excellent transparency and heat resistance, but have poor mechanical strengths and high gas permeability. As a result, brightness of LEDs becomes lower due to sulfurization of electrodes, which is a drawback.
A composition comprising a vinyl resin having T unit, i.e., PhSiO3/2, and an MQ resin having an SiH group is known to lower a gas permeability in the following patent literature 4. A cured product of this composition has sufficient mechanical strengths, but shows poor heat resistance, and discoloration in a long term of heating, which is a drawback.
A composition comprising a silicone resin composed of M units, D units and Q units is also known in the following patent literature 5. This composition is good in handling properties and self-adhesion property. However, a cured product of this composition has an insufficient refractive index, insufficient mechanical strengths and a large gas permeability.